Dispatch
Knowledge of Bat Rabies
and Human Exposure Among United States Cavers
Robert V. Gibbons,* Robert C. Holman,* Stephen R. Mosberg,†
and Charles E. Rupprecht*
*Centers for Disease Control and Prevention, Atlanta, Georgia,
USA; and †National Speleological Society, Huntsville, Alabama, USA
We surveyed
cavers who attended the National Speleological Society convention
in June 2000. Fifteen percent of respondents did not consider
a bat bite a risk for acquiring rabies; only 20% had received
preexposure prophylaxis against the disease. An under-appreciation
of the risk for rabies from bat bites may explain the preponderance
of human rabies viruses caused by variant strains associated with
bats in the United States.
Over the past century, human rabies has become exceedingly rare
in the United States. The decreasing incidence of human rabies has
followed the decline of rabies in domestic dogs. From 1946 to 1965,
236 human Rabies virus (RABV) infections were reported in
the United States. From 1946 through 1949, the number of human RABV
infections averaged 24/year, declining to 1.5/year from 1962 through
1965. Ninety percent of RABV infections were caused by dog bites
from 1946 through 1949, decreasing to 67% from 1962 through 1965
(1). As canine rabies declined, the relative importance
of other reservoirs in the United States increased. From 1970 to
1989, human infections averaged 3.3/year. Of these infections, 45%
were caused by canine RABV variants (all but one was acquired outside
the United States), 30% were caused by bat RABV variants, and one
was caused by a corneal transplant from an unsuspected rabies patient
(2,3). From 1990 through 2000, bat RABV variants
have emerged as the predominant cause of human rabies in the United
States (4). In the past 11 years, total human rabies
deaths have averaged 2.9/year, and 24 (75%) of 32 deaths were due
to bat RABV variants. If the six cases caused by foreign canine
RABV variants are excluded, then 24 (92%) of the 26 human rabies
deaths acquired domestically were caused by bat RABV variants. The
other two cases were due to a dog/coyote RABV variant found in Texas
(4).
Confusion remains about potential exposures to rabies from bats.
Only 2 (8%) of the 24 patients with human rabies caused by bat RABV
variants had a definitive history of a bat bite. Nine patients (38%)
had a history of direct physical contact with bats, 5 (21%) had
a history of a bat inside the living area, and 8 (33%) had no history
of proximity to bats (4). Because of the paucity
of bat (or other animal) bite histories, could these human rabies
cases have been acquired through aerosol transmission? The diagnosis
of rabies in two people who had no known history of a bite, but
who worked extensively in caves inhabited by bats, received considerable
attention in 1953 (1,2). Although the aerosol route
is considered a possible mechanism of RABV acquisition, few data
support such transmission under typical field conditions. A more
plausible hypothesis is that many people may not be aware that a
bat bite is a risk for rabies transmission and fail to report it.
Because of the potential contact with bats, cavers are considered
at a higher risk for rabies exposure than the general population.
Since the 1960s, the recommendation has been that the cavers receive
rabies preexposure prophylaxis (PreEP) (5). The
objectives of this study were to learn about cavers’ knowledge of
the risks for bat-to-human rabies transmission and to quantify cavers’
use of rabies PreEP prophylaxis and postexposure prophylaxis (PostEP).
The
Study
We administered a survey to cavers attending the National Speleological
Society Convention in Elkins, West Virginia, USA, in June 2000.
The survey was included in the convention registration packet. Verbal
reminders to return the survey were given, and collection boxes
were located at several sites at the convention.
The survey asked respondents about demographic information, how
long and how many times they had been caving, how often they encountered
bats when caving, if they had been advised to receive the rabies
PreEP and if they had received it, if they considered specific scenarios
(bat bite, bat scratch, bat on skin, bat on clothing, indirect contact
with bats) as a potential risk for rabies, if they had ever had
a potential exposure to rabies, and if they had ever received rabies
PostEP.
Categorical variables were compared using the chi-square test or
the Fisher’s exact test (2-tailed), as appropriate. Continuous variables
were analyzed with the Wilcoxon rank-sum test (6).
Multivariate logistic regression was used for multivariate analysis.
Questionnaires were returned from 392 (26%) of 1,508 cavers attending
the convention. The respondents’ mean age was 47 (range 12-84) years,
68% were male, and 76% were college graduates. The respondents caved
a mean of 23 (range 1-58) years and a mean of 16 (range 0-150) times
in the past year. When asked how often they see bats on their caving
trips, 1% responded never, 29% sometimes, 22% about half the time,
43% often, and 5% always. Respondents were asked to address whether
specific scenarios with bat(s) were considered a risk for rabies
(Table 1).
The respondents who thought a bat bite was not a risk for rabies
were younger (43 versus 48 years, p=0.009) and less educated (43%
versus 21% were not college graduates, p=0.005) but did not differ
significantly by gender, number of years caving, or number of times
caving in the past year. The respondents who thought that indirect
contact with bats was a risk for rabies were older (52 versus 46
years, p<0.001), and caved more years (28 versus 22; p<0.001).
They did not significantly differ by gender, education, or number
of times caving in the past year. Seventy-six (20%) respondents
received PreEP (Table 2). In multivariate
analysis, having been advised to receive the vaccine was independently
associated with having received it (odd ratio = 31; 95% confidence
interval 15 to 61).
Eighty-eight (23%) respondents had been advised to receive PreEP.
Those who caved more years (25 versus 22, p=0.05), and more times
in the last year (25 versus 15, p<0.001) were more likely to
have been advised to have PreEP. College graduates were more likely
to be advised to have PreEP, but statistical significance was not
found (24% versus 17%, p=0.14). Those advised to get PreEP did not
differ by age or gender. Of the 66 respondents advised to get PreEP
because of caving, 37 (57%) had done so; of the 20 advised to get
PreEP for other reasons, 17 (85%) had done so. Twenty-four (1.6%)
respondents felt they had been potentially exposed to rabies. Of
the 24, only 5 involved exposures to bats (3 from bites), and only
1 indicated this exposure was directly associated with caving.
Conclusions
Despite the cavers’ education level and their familiarity with
bats, 14% of the cavers did not consider a bat bite risk for rabies.
When only the cavers without a college degree were considered, 26%
did not think a bat bite was a risk for rabies. Given the general
public’s assumed education level and overall lack of familiarity
with bats, the percentage of the public who do not consider a bat
bite a risk for rabies is probably higher than (or closer to) 26%,
than 14%. If so, this would support the hypothesis that people may
lack the knowledge to seek medical care if a bat bites them. Unlike
bites from larger mammalian carnivores, lesions resulting from a
bat bite probably will not warrant seeking medical care. In addition,
39% of cavers did not think a bat scratch was a risk for rabies.
Technically, a scratch contaminated with saliva is an exposure,
but scratches alone are less likely to transmit rabies than a bite.
The practical problem arises in the consideration of scratches from
bats. Does the patient know if the scratch is contaminated with
saliva? And more importantly, can a patient discern a scratch from
a bite, particularly under the darkened and tight recesses of a
cave?
Eleven percent of cavers felt that indirect contact with bats to
be risk for rabies. Some cavers (especially older, more experienced
members) may possess knowledge of those rare cases of human rabies
that are attributed to aerosol transmission. Two infections in the
1950s, commonly attributed to aerosol transmission in crowded bat
caves (in a bat researcher and a mining engineer), had other possible
mechanisms of infection (7,8), and no other infections
have been reported in cavers. Interestingly, the lack of rabies
cases in cavers is evidence against the occurrence of aerosol transmission,
except under extraordinary circumstances. The respondents in our
study, if projected to only cavers who are members of the NSS, represent
over 4 million caving episodes; nearly 60% involved cavers with
no PreEP. Of course, the expected prevalence of rabies in freeranging
bats is low, probably <1% (9).
This survey is limited by a low response rate and may be subject
to selection bias. Those who did respond may be more or less familiar
with rabies than the average caver. In addition, the survey may
be subject to response bias. Relationships demonstrated are associations;
cause and effect cannot be definitively determined.
Nevertheless, our study suggests that despite longstanding guidelines
for cavers to receive PreEP for rabies, only 20% have done so. The
increase is modest when compared to a survey conducted in 1970 of
239 cavers, which found that only 14% had received PreEP (CDC, unpub.
data). Increasing the cavers’ awareness about the recommendation
may increase compliance, as 64% of those advised to receive PreEP
had done so, compared to 6% (n=19) of those not advised to do so.
In fact, this was the only independent predictor of receiving PreEP.
A future survey of the general public is indicated to explore their
knowledge and attitudes towards bats, rabies, and the risk for acquisition.
Acknowledgments
We thank Taber Gibbons for his help in collecting data, John O’Connor
for editorial assistance, the staff in the Viral and Rickettsial
Zoonoses Branch for useful comments, and the participants from the
National Speleological Society for their support.
Dr. Gibbons is a medical officer with the Department of Virus Diseases
at the Walter Reed Army Institute of Research. His main area of
interest is dengue vaccines.
References
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- Anderson LJ, Nicholson KG, Tauxe RV, Winkler WG. Human
rabies in the United States, 1960 to 1979: epidemiology, diagnosis,
and prevention. Ann Intern Med 1984;100:728-35.
- Noah DL, Drenzek CL, Smith JS, Krebs JW, OrclariL, Shaddock,
et al. Epidemiology
of human rabies in the United States, 1980 to 1996. Ann Intern
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- Human rabies—California, Georgia, Minnesota, New York, and Wisconsin,
2000. MMWR Morb Mortal Wkly Rep. 2000;49:1111-5.
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Pre-exposure rabies prophylaxis in amateur spelunkers. J Am
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Table
1. The number of cavers who considered the scenario as a
risk for rabies |
|
Scenario
|
College degree or higher (n=298)
|
No college degree (n=94)
|
Total (%)
|
|
Bat bite
|
262/294 (89)
|
69/93 (74)a
|
331/387 (86)
|
Bat scratch
|
191/290 (66)
|
42/92 (46)a
|
233/382 (61)
|
Bat on skin
|
42/292 (14)
|
9/93 (10)
|
51/385 (13)
|
Bat on clothing
|
10/293 (3.4)
|
1/93 (1.1)
|
11/386 (2.9)
|
Being around batsb
|
37/293 (13)
|
8/93 (8.6)
|
45/386 (12)
|
|
aFor having college degree or higher
compared to no college degree, p<0.001.
bIndirect contact with bats.
|
Table
2. Number of cavers who had/had not received preexposure
prophylaxis (PreEP) |
|
Characteristic
|
Received (n = 76)
|
Not received (n = 313)
|
p value
|
|
College
graduate |
63/76 (83%)
|
231/311 (74%)
|
0.12
|
Advised to get PreEPa
|
56/75 (75%)
|
31/315 (10%)
|
<0.001
|
Male gender
|
55/76 (72%)
|
210/312 (67%)
|
ns
|
See bats > half of the time
|
57/71 (80%)
|
211/311 (68%)
|
0.04
|
Mean age
|
49 yr
|
46 yr
|
ns
|
Mean years caving
|
26 yr
|
22 yr
|
0.01
|
Mean times caving
|
34/yr
|
13/yr
|
< 0.001
|
|
aThe only variable independently
associated with receiving PreEP.
|
|